1. Field of the Invention
The present invention relates to an image sensor module used for an image reader. The present invention further relates to a light guiding member used for such an image sensor module.
2. Description of the Related Art
Conventionally, image readers are known such as a document scanner for reading an image printed on a document or a bill reader for discriminating between real bills and counterfeit bills. As a unit for constituting such an image reader, an image sensor module having the function to illuminate the object to be read with linear light and the function to read the reflected light is widely used.
FIG. 11 is a sectional view showing the structure of a conventional image sensor module X (see JP-A-2000-125080). The image sensor module X includes a case 91 elongated in a direction perpendicular to the sheet surface of FIG. 11. A reflector 92, a light guiding member 93, a lens unit 94, a substrate 95, an optical sensor 96 and an LED module (light source) are accommodated in the case 91. The light guiding member 93 and the optical sensor 96 extend in the longitudinal direction of the case 91.
The light emitted from the LED module travels in the light guiding member 93 in the longitudinal direction. The light guiding member 93 includes a reflecting portion 93a and a light emitting surface 93b. The light traveling in the light guiding member 93 is reflected by the reflecting portion 93a toward the light emitting surface 93b, so that linear light extending in the longitudinal direction is emitted from the light emitting surface 93b. The linear light is reflected by the object P to be read, and the reflected light is converged by the lens unit 94 onto the optical sensor 96. The optical sensor 96 outputs the image data corresponding to the received amount of light.
The conventional image sensor module X still has room for improvement in the following points. That is, in the image sensor module X, the reading accuracy of data is relatively low at portions adjacent to the ends in the longitudinal direction. One of the reasons for this is that the luminance distribution of the linear light emitted from the light guiding member 93 is not necessarily uniform. Specifically, although the luminance of the linear light from the light guiding member 93 is relatively uniform at portions adjacent to the center, the luminance decreases as progressing toward the ends.
As another reason for the relatively low reading accuracy of data, the luminance of the linear light reflected by the object P changes locally before the light reaches the optical sensor 96. For instance, it is now assumed that linear light with uniform luminance distribution is directed to an object P the entire surface of which is uniformly white. Ideally, in this case, the luminance of the reflected light to reach the optical sensor 96 should be uniform along the longitudinal direction of the sensor. In the conventional image sensor module X, however, the luminance of the reflected light tends to become non-uniform in the stage of entering the lens unit 94.